The present invention relates to magnetic resonance imaging (MRI) systems and, in particular, to the radio-frequency (RF) coils used in such systems.
U.S. Patent Documents:                U.S. Pat. No. 5,363,845, Breast coil for magnetic resonance imaging, Chowdhury and Boskamp        U.S. Pat. No. 5,386,447, Mammographic screening and biopsy apparatus, Siczek        U.S. Pat. No. 5,416,413, Magnetic resonance examination apparatus comprising a coil system for MR mammography) Leussler        U.S. Pat. No. 5,437,280, Magnetic resonance breast localizer, Hussman        U.S. Pat. No. 5,602,557, Mammography antenna arrangement for NMR examinations of a female breast, Duerr        U.S. Pat. No. 5,804,969, MRI RF coil Lian and Roemer        U.S. Pat. No. 6,163,717, Open structure breast coil and support arrangement for interventional MRI, Su        U.S. Pat. No. 6,198,962B1, Quadrature detection coil for interventional MRI, Su        U.S. Pat. No. 5,706,812, Stereotactic MRI breast biopsy coil and method for use, Strenk et al.        
The above-cited documents and references are incorporated herein by reference.
Magnetic resonance imaging (MRI) utilizes hydrogen nuclear spins of the water molecules in a human body, which are polarized by a strong, uniform, static magnetic field generated by a main magnet (named B0−the main magnetic field in MRI physics). The magnetically polarized nuclear spins generate magnetic moments in a human body. The magnetic moments point in the direction of the main magnetic field in a steady state, and produce no useful information if they are not disturbed by any excitation.
The generation of a nuclear magnetic resonance (NMR) signal for MRI data acquisition is accomplished by exciting the magnetic moments with a uniform radio frequency (RF) magnetic field (named B1 field or the excitation field). The B1 field is produced in the imaging region of interest by an RF transmit coil which is driven by a computer-controlled RF transmitter with a power amplifier. During excitation, the nuclear spin system absorbs magnetic energy, and its magnetic moments precess around the direction of the main magnetic field. After excitation, the precessing magnetic moments will go through a process of free induction decay (FID), releasing their absorbed energy and returning to the steady state. During free induction decay, NMR signals are detected by the use of a receive RF coil, which is placed in the vicinity of the excited volume of the human body. The NMR signal is the secondary electrical voltage (or current) in the receive RF coil that has been induced by the precessing magnetic moments of the human tissue. The receive RF coil can be either the transmit coil itself, or an independent receive RF coil. The NMR signal is used for producing images by using additional pulsed magnetic gradient fields, which are generated by gradient coils integrated inside the main magnet system. The gradient fields are used to spatially encode the signals and selectively excite a specific volume of the human body. There are usually three sets of gradient coils in a standard MRI system, which generate magnetic fields in the same direction as the main magnetic field, varying linearly in the imaging volume.
In MRI, it is desirable for the excitation and reception to be spatially uniform in the imaging volume for better image uniformity. In a standard MRI system, the best excitation field homogeneity is usually obtained by using a whole-body volume RF coil for transmission. The whole-body transmit coil is the largest RF coil in the system. A large coil, however, produces lower signal-to-noise ratio (SNR or S/N) if it is also used for reception, mainly because of its greater distance from the signal-generating tissues being imaged. Since a high signal-to-noise ratio is one of the most desirable characteristics in MRI, special-purpose coils are often used for reception to enhance the S/N ratio from the volume of interest.
In practice, a well-designed specialty RF coil should have the following functional properties: a high S/N ratio, good uniformity, high unloaded Quality factor (Q) of the resonance circuit, and a high ratio of the unloaded to loaded Q factors. In addition, the coil device should be mechanically designed to facilitate patient handling and comfort, and to provide a protective barrier between the patient and the RF electronics. Another way to increase the SNR is by quadrature reception. In this method, NMR signals are detected in two orthogonal directions, which are in the transverse plane or perpendicular to the main magnetic field. The two signals are detected by two independent coils/receivers that cover the same volume of interest. In this quadrature mode, the SNR can be increased by up to a factor of the square root of 2 over that of the individual linear coils.
Breast cancer is one of the leading causes of death in today's women. Early diagnosis of malignancy and follow-up therapy by noninvasive MRI/MRS could greatly enhance survival chances. It is also noted that biopsy or interventional applications are often performed on a female patient to treat cancer tissues or diagnose suspicious lesions within a breast. For this reason, it is desirable to design breast coils that are customized to the size and shape of individual breasts in order to obtain a maximized filling factor for the volume of interest while providing an easy and open access to breasts for a surgeon to perform biopsy/interventional procedures.
For conventional breast MRI, a whole body coil is typically used as a transmitter coil and a smaller receiver coil is utilized to receive signals from breast tissues being examined. However, a whole body coil is not necessarily optimized or customized for breast imaging, in general, since it produces a uniform excitation around the center of the coil, this usually does not coincide with the position of breasts. Another disadvantage in common with many breast coils is that they do not provide an easy and open access for a surgeon to perform biopsy/interventional procedures on a patient.